Extracts of brains of newborn mice contain a factor which can inhibit murine neuroblastoma cell growth in vitro. This factor is stable to heat, has an apparent molecular weight of about 700, and is susceptible to the action of pronase. The factor appeared to cytotoxic to the neuroblastoma cells at the concentration tested. It was equally inhibitory towards a murine glioma cell line but less effective on rat glioma cells. The factor had no significant inhibitory effect on the growth of rat hepatoma, murine melanoma, or transformed murine fibroblasts. Experiments to isolate this factor will be continued. Ongoing studies of the ganglioside composition of human neuroblastomas suggest that a lack of GT1b may correlate with a poor prognosis. In the approximately 60 different neuroblastoma samples assayed, we have observed both qualitative and quantitative differences in pattern. (M)